cells arise from other cells

Question 1

state what the cell cycle is and outline stages

Answer 1

cycle of division with intermediate growth periods
1. interphase
2. mitosis or meiosis
3. cytokinesis

Question 2

explain why the cell cycle does not occur in some cells

Answer 2

after differentiation, some types of cell in multicellular organisms no longer have the ability to divide

Question 3

what is the difference between the cell cycle and mitosis

Answer 3

cell cycle includes growth period between divisions; mitosis is only 10% of the cycle and refers only to nuclear division

Question 4

outline what happens during interphase

Answer 4

G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres and cell size doubles

S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a chromatid

G2: organelles divide

Question 5

state the purpose of mitosis

Answer 5

produces 2 genetically identical daughter cells for growth, cell replacement, tissue repair and asexual reproduction

Question 6

name the stages of mitosis

Answer 6

prophase
metaphase
anaphase
telophase

Question 7

outline what happens during prophase

Answer 7

1. chromosomes condense, becoming visible (X-shaped: 2 sister chromatids joined at centromere)
2. centrioles move to opposite poles of cell and mitotic spindle fibres form
3. nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 8

outline what happens during metaphase

Answer 8

sister chromatids line up at cell equator attached to the mitotic spindle by their centromeres

Question 9

outline what happens during anaphase

Answer 9

requires energy from ATP hydrolysis

1. spindle fibres contact = centromeres divide
2. sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of cell
3. spindle fibres break down

Question 10

outline what happens during telophase

Answer 10

1. chromosomes decondense, becoming invisible again
2. new nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome

Question 11

explain the procedure for a root tip squash experiment

Answer 11

1. prepare a temporary mount of root tissue
2. focus an optical microscope on the slide. count total number of cells in the field of view and number of cells in a stage of mitosis
3. calculate mitotic index (proportion of cells undergoing mitosis)

Question 12

explain how to prepare temporary root tip mount

Answer 12

1. place root in hydrochloric acid to halt cell division and hydrolyse middle lamella
2. stain root tip with a dye that binds to chromosomes
3. macerate tissue in water using mounted needle
4. use mounted needle at 45 degrees to press down coverslip and obtain a single layer of cells. avoid trapping air bubbles

Question 13

name 2 dyes that bind to chromosomes

Answer 13

toluidine blue (blue)

acetic orcein (purple-red)

Question 14

why is only the root tip used when calculating a mitotic index

Answer 14

• meristematic cells at root tip are actively undergoing mitosis
• cells further from root tip are elongating rather than dividing

Question 15

what are tumour suppressor genes

Answer 15

genes that code for proteins to trigger apoptosis / slow cell cycle

Question 16

what are proto-oncogenes

Answer 16

genes that code for proteins to stimulate cell cycle to progress from one stage to the next

Question 17

how can mutation to tumour suppressor genes and proto-oncogenes cause cancer

Answer 17

• tumour suppressor - no production of a protein needed to slow cell cycle
• proto-oncogenes - form permanently-activated oncogenes
• disruption to cell cycle - uncontrolled cell division = tumour

Question 18

suggest how cancer treatments control rate of cell division

Answer 18

disrupt cell cycle:
- prevent dna replication
- disrupt spindle formation = inhibit metaphase / anaphase

can also damage healthy cells

Question 19

how do prokaryotic cells replicate

Answer 19

binary fission

1. dna loop replicates, both copies stay attached to cell membrane, plasmids replicate in cytoplasm
2. cell elongates, separating the 2 dna loops
3. cell membrane contracts and septum forms
4. cell splits into 2 identical progeny cells, each with 1 copy of the dna loop but a variable number of plasmids

Question 20

why are viruses classified as non-living

Answer 20

they are cellular: no cytoplasm, no metabolism and cannot self replicate

Question 21

outline how viruses replicate

Answer 21

1. attachment proteins attach to receptors on host cell membrane
2. enveloped viruses fuse with cell membrane or move in via endocytosis and release dna/rna into cytoplasm or viruses inject dna/rna
3. host cell uses viral genetic information to synthesise new viral proteins/nucleic acid
4. components of new viral particle assemble

Question 22

how do new viral particles leave the host cell

Answer 22

a) bud off and use cell membrane to form envelope
b) cause lysis of host cell